Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs).
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations move among BTS coverage areas, and in doing so encounter varying radio propagation environments.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range where the BTS can maintain acceptable communications with MSs operating within its serving cell. Coverage areas for a plurality of BTSs can be aggregated for an extensive coverage area. An embodiment of the present invention is described with reference to the Third Generation Partnership Project (3GPP) defining portions of the Universal Mobile Telecommunication Standard (UMTS), including the time division duplex (TD-CDMA) mode of operation. 3GPP standards and technical release relating to the present invention include 3GPP TR 25.211, TR 25.212, TR 25.213, TR 25.214, TR 25.215, TR 25.808, TR 25.221, TR 25.222, TR 25.223, TR 25.224, TR 25.225, TS 25.309, TR25.804, TS 21.101, and TR 21.905 hereby incorporated within this application, in their entireties by reference. 3GPP documents can be obtained from 3GPP Support Office, 650 Route des Lucioles, Sophia Antipolis, Valbonne, FRANCE, or on the Internet at www.3gpp.org.
In UMTS terminology, a BTS is referred to as a Node B, and subscriber equipment (or mobile stations) are referred to as user equipment (UEs). With the rapid development of services provided to users in the wireless communication arena, UEs can encompass many forms of communication devices, from cellular phones or radios, through personal data accessories (PDAs) and MP-3 players to wireless video units and wireless internet units.
In UMTS terminology, the communication link from the Node B to a UE is referred to as the downlink channel. Conversely, the communication link from a UE to the Node B is referred to as the uplink channel.
In such wireless communication systems, methods for simultaneously using available communication resources exist where such communication resources are shared by a number of users (mobile stations). These methods are sometimes termed multiple access techniques. Typically, some communication resources (say communications channels, time-slots, code sequences, etc) are used for carrying traffic while other channels are used for transferring control information, such as call paging, between the Node Bs and the UEs.
It is worth noting that transport channels exist between the physical layer and the medium access control (MAC) in the system hierarchy. Transport channels can define how data is transferred over the radio interface. Logical channels exist between MAC and the radio link control (RLC)/radio resource control (RRC) layers. Logical channels define what is transported. Physical channels define what is actually sent over the radio interface, i.e. between layer 1 entities in a UE and a Node B.
A number of multiple access techniques exist, whereby a finite communication resource is divided according to attributes such as: (i) frequency division multiple access (FDMA) in which one of a plurality of channels at different frequencies is assigned to a particular mobile station for use during the duration of a call; (ii) time division multiple access (TDMA) whereby each communication resource, say a frequency channel used in the communication system, is shared among users by dividing the resource into a number of distinct time periods (time-slots, frames, etc.); and (iii) code division multiple access (CDMA) whereby communication is performed by using all of the respective frequencies, at all of the time periods, and the resource is shared by allocating each communication a particular code, to differentiate desired signals from undesired signals.
Within such multiple access techniques, different duplex (two-way communication) paths are arranged. Such paths can be arranged in a frequency division duplex (FDD) configuration, whereby a frequency is dedicated for uplink communication and a second frequency is dedicated for downlink communication. Alternatively, the paths can be arranged in a time division duplex (TDD) configuration, whereby a first time period is dedicated for uplink communication and a second time period is dedicated for downlink communication on an alternating basis.
Present day communication systems, both wireless and wire-line, have a requirement to transfer data between communications units. Data, in this context, includes signaling information and traffic such as data, video, and audio communication. Such data transfer needs to be effectively and efficiently provided for, in order to optimize the use of limited communication resources.
Recent focus in 3GPP has been on the introduction and development of an “enhanced uplink” feature to provide fast scheduling and allocation of system resources for uplink packet-based data, and to serve as a compliment to HSDPA (high-speed downlink packet access). Within HSDPA (downlink), a scheduling (or downlink resource allocation) entity is placed in the Node-B network entity (previously scheduling was performed by a Radio network controller, RNC). The scheduler resides within a new Medium Access Control (MAC) entity termed the MAC-hs (the “hs” denoting that the MAC entity is associated with HSDPA). Similarly, for enhanced uplink, the uplink scheduler has also been moved from the RNC (where it resided in pre-enhanced-uplink implementations) into a new MAC entity termed MAC-e, located within the Node-B.
For HSDPA (downlink) and enhanced uplink, scheduling is generally distributed among Node-Bs such that an uplink and downlink scheduler exists in each cell which is largely, or wholly unaware of scheduling decisions made in other cells. Schedulers in different cells typically operate independently, although schedulers for cells served by the same Node-B (same basestation) may cooperate. In some implementations there may also be cooperation between uplink and downlink schedulers. Feedback is provided to the uplink and downlink schedulers from the UE pertaining to current radio conditions and this information is used by the schedulers to adjust parameters of the uplink or downlink radio links. Examples of link parameters which may be adjusted in response by the scheduler to maintain an acceptable quality or reliability of radio communication between the base station and the UE. include: (i) the data rate; (ii) the transmit power; (iii) the modulation format (e.g. QPSK/8-PSK/16-QAM); and (iv) the degree of FEC coding applied
In the case of enhanced uplink, control of the UE transmission power and data rate takes the form of grant commands sent from one or more uplink schedulers to the same UE. The absolute grant channel (E-AGCH) is used by the serving cell scheduler to convey information to the UE about which resources it may use. Uplink resources are generally thought of in CDMA systems as “Rise-over-Thermal” (RoT) resources wherein an allowable received-interference level threshold is set for the base station (relative to thermal noise in the receiver) and each user is effectively granted a fraction of this allowable received interference power. As the allowable RoT set-point is increased, so the interference level at the base station increases and the harder it becomes for a UEs signal to be detected. Thus, the consequence of increasing the RoT is that the coverage area of the cell is reduced. The RoT set-point must therefore be configured correctly for a given deployment to ensure the desired system coverage is met.
It is then also clear, that accurate control of the RoT and other system resources granted to active users in the system is critical for efficient system operation. Grants of resources to UE's which are not accurately tailored to the UE's radio conditions or data transmission needs are wasteful of system resources.
It is therefore important that the uplink scheduler is informed and updated with minimal delay as to any change in each active UE's radio conditions or data transmission requirements. For example, if a user who has been relatively idle in recent times tries to send an email attachment, it is important that the uplink scheduler is made aware of the UE's requirement to send data with minimal delay such that system resources can be granted and a fast response time is achieved. It is also beneficial if the uplink scheduler is made aware of the current radio conditions for the UE such that the nature and parameters of the assigned transmission resources can be adjusted to suit those radio conditions.
For the purposes of conveying this type of request for transmission resources, and any associated updates of radio conditions, from the UE to the enhanced uplink scheduler in Node-B, a new Node-B terminated random access channel is considered, termed the E-PRACH. This physical channel is used within a conceptual enhanced uplink system extension for 3GPP UTRA TDD to carry a timely indication to the Node-B scheduler of a users need for allocation of uplink shared transmission resources.
The random access channels (physical random access channels, “PRACH,” in 3GPP terminology) work as follows: When the need for transmission is identified in the user equipment, a transmission channel or “code” is selected at random from the set of configured channels. Transmission is then made on that channel and it is hoped that no other user transmits on the same channel at the same time; this results in a collision and typically detection or decoding failure of all colliding users on that channel.
The probability of collision increases with the transmission probability and with a decreasing number of configured channels. Usage of the E-PRACH channel should not be overly frequent due to the fact that it should only be required at the start of each packet call, (ie: following short periods of inactivity). However, a sufficient amount of code resource (“NE” E-PRACH channels) should be assignable for E-PRACH usage such that the probability of E-PRACH collisions between users is maintained at a suitably low level. Fewer assigned E-PRACH channels will result in an increased probability of collision for a given system load. A high probability of E-PRACH collision will degrade transmission latency and ultimately, perceived user throughput because the necessary information regarding a users need for transmission resources may be delayed or lost on transit from the UE to the Node-B scheduler.
Existing releases (eg: release 99/4/5/6) of 3GPP UTRA TDD already provide support for a physical random access channel (termed here R99 PRACH). Again, an appropriate number (“N99”) of R99 PRACH channels must be configured to maintain acceptable probability of collision for a given system load. Mechanisms are also provided for the network to control the collision probability via cell broadcast control signaling of so-called “RACH-persistence” values. These are used in each user terminal to adjust the probability of RACH transmission. Thus at high loads, the network can command UEs to reduce transmission of RACH in an attempt to maintain acceptable collision probability.
For a given number of total random access channels (NTOT) the overall collision probability for both E-PRACH and R99 PRACH may be reduced if the entire available set of random access channels is used simultaneously for both E-PRACH and R99 PRACH transmissions.
Correspondingly, if the total available set of random access channels (NTOT) is divided into a set that may be used for R99 PRACH and a non-intersecting set that may be used for E-PRACH, then the collision probability for both R99 PRACH and E-PRACH is degraded when compared to the case where the set of random access channels is not segmented and is instead shared between the two PRACH types.
This “channel sharing” capability also allows for an accommodation of changes in the volume of offered PRACH traffic for each PRACH type over time. For example, if during some hours of service, 80% of PRACH's are R99 type and 20% are E-type, then a system implementing a hard-split in random access channel resources would need to reconfigure those resources such that an appropriate amount of the resource was assigned to each PRACH type. If such reconfiguration was not performed then the system would suffer from suboptimal PRACH capacity on one of the PRACH types. Reconfiguration would need to be performed each time a change in the offered PRACH traffic volume ratio (R99/E-PRACH) was detected. On the other hand, if the resources were shared between PRACH types then the means for such a reconfiguration need not exist.
It is therefore desirable to allow E-PRACH and R99 PRACH to transmit on a common set of available random access channels 102b in FIG. 1b. Although this is capable of improving both R99 PRACH and E-PRACH efficiency, this approach does introduce the problem of how to reliably distinguish an E-PRACH from a PRACH at the base station receiver. The problem arises because different actions need to be taken by the base station on receipt of a R99 PRACH or an E-PRACH. The R99 PRACH is terminated at the radio network controller, “RNC” (MAC-c/sh) and as such any detected R99 PRACH should be decoded by the base station and forwarded over the Iub interface between a Node-B and the RNC. On the other hand, E-PRACH is terminated at Node-B (MAC-e) and as such should not be forwarded over Iub; instead the information contained within it should be forwarded internally within the base station to the uplink scheduler in MAC-e.
The situation in which the random access resources are segregated into resources exclusively assigned to R99 PRACH, and resources exclusively assigned to E-PRACH is depicted in FIG. 1a. User Equipment 101 can originate R99 PRACH and E-PRACH data packets in its MAC-c/sh and MAC-e medium access control entities, respectively. R99 PRACH and E-PRACH data packets are both sent through the physical layer 107 of UE 101, where they are assigned transmission resources on the air interface 102 depending on the PRACH type (E-PRACH/R99-PRACH), and are received by the physical layer 108 of Node-B 103. The PRACH type is determined by the Node-B simply by the random access channel the signal was received on. R99 PRACH data packets are then forward over the Node-B/RNC interface Iub 104 to RNC 105, where they are terminated in the MAC-c/sh 110. E-PRACH data packets are forwarded internally to the Node-B from the Node-B physical layer to the Node-B MAC-e 109.
It is desirable to avoid the segregation of random access resources into sets of channels exclusively for use by a single PRACH type, and instead to allow PRACH types to share a common set of random access resources. For backward compatibility with legacy Node-Bs that don't have E-PRACH capability, it is also desirable to be able to introduce E-PRACH into existing systems without requiring modification to R99 PRACH data packets.